A healthy human heart may be able to maintain its own intrinsic rhythm often for the lifetime of the patient. Yet, even a healthy human heart will sometimes exhibit some irregularities, such as premature ventricular contractions (PVCs) and paroxysmal atrial tachycardias (PATs), among others, that upset the heart's AV sequential contractions, momentarily diminishing cardiac output. Some of these momentary interruptions in cardiac rhythm seldom require medical intervention. On the other hand, individuals may be born with or develop cardiac arrhythmias that require medical intervention. Some of these patients may benefit from drug therapy, while others will require an implantable medical device (IMD), such as a pacemaker or a pacer cardiac defibrillator (PCD), among others.
Generally, pacers are used to treat bradyarrhythmias, whereas PCDs are used to treat tachyarrhythmias. By properly timing the delivery of pacing pulses, the heart will contract in proper rhythm, thereby restoring its efficiency as a pump. PVCs or PACs, however, may occur in some patients and interrupt the paced cardiac rhythm. Such interruptions may, at times, result in atrial or ventricular tachyarrhythmias. Atrial fibrillation may, in turn, induce irregular ventricular heart rhythms by processes not yet fully understood. Such induced ventricular arrhythmias compromise pumping efficiency even more drastically than atrial arrhythmias.
PCDs are used with patients who already exhibit signs of an irritable heart. PCDs differentiate between various types of ventricular tachycardias (VTs), such as non-life-threatening VTs and life-threatening ventricular fibrillation (VF), and treat them differently. With these devices it is even more important to prevent the development of a VT, since these arrhythmias can lead to a life-threatening arrhythmia.
Other cardiac rhythm management systems treat congestive heart failure (CHF). Congestive heart failure can be treated by biventricular, coordinated pacing therapy that provides pacing pulses to both right and left ventricles in a sequential fashion. Such devices also must ensure against any irregularities in rhythm that sometimes occur due to interactions between the device and the heart. Even though a congestive heart failure patient may have adequate ventricular coordination and cardiac output in the presence of a normal sinus rhythm, when an atrial or ventricular premature event occurs, biventricular coordination may cease, seriously worsening cardiac function.
Rate smoothing algorithms are not new to the art of preventing arrhythmias or to the maintenance of AV synchrony. Such software systems have been implemented in the Medtronic® Gem® DR with ventricular rate stabilization in the DDD/R modes and the Medtronic® Jewel® AF with atrial rate stabilization in the DDD mode.
Over the years, a variety of pacing modes have been developed to respond to changes in spontaneous heart activity. One of the earliest of these is disclosed in U.S. Pat. No. 3,857,399, issued to Zacouto. Zacouto provides a system in which the underlying heart rhythm is used to control the onset of pacing, but not the pacing rate. The onset of pacing in Zacouto can occur after an interval either shorter or longer than the underlying physiologic interval.
U.S. Pat. No. 4,163,451 issued to Lesnick et al discloses a pacemaker having an overdrive-pacing mode. It provides for initiation of cardiac pacing at an interval shorter than the detected cardiac interval. The purpose of this pacing modality is to provide a method of tachycardia treatment.
U.S. Pat. No. 4,562,841 issued to Brockway et al discloses a dual chamber pacemaker, in which a gradual increase and decrease of pacing intervals occurs in response to changes in the underlying heart rate. However, adjustment of the pacing interval is primarily based on the atrial rate, rather than the ventricular. There appears to be no provision, however, for adjusting the atrial escape interval as a function of the measured escape interval following a PVC.
U.S. Pat. No. 4,503,857 issued to Boute et al discloses a cardiac pacemaker that drops the ventricular rate to the lower limit of a “physiologic rate band” and thereafter gradually lengthens the escape interval in response to an equation until a programmed rate is attained. This modality is referred to as “flywheel” pacing, and is intended to prevent abrupt changes in pacing rate.
U.S. Pat. No. 4,467,810 issued to Vollmann discloses a dual chamber pacemaker that employs a fall-back pacing mode in which ventricular pacing intervals are gradually incremented in response to a high atrial rate. The alteration of escape intervals is intended to terminate atrial tachycardias.
In U.S. Pat. No. 4,941,471, Mehra discloses a cardiac pacemaker which generates stimulus pulses and senses the occurrence of natural heartbeats in a patient. The pacemaker provides for a mode of pacing that tracks naturally conducted depolarizations, and responds to PVCs by gradually increasing pacing cycle until a predetermined lower rate is reached. In U.S. Pat. No. 5,814,085, Hill reveals a cardiac pacemaker with a rate stabilization pacing mode. The pacemaker varies the increment following a cycle ending in a sensed depolarization as a function of the underlying heart rate and may additionally vary the increment as a function of the prematurity of the most recently sensed depolarization relative to the preceding depolarization.
Denker, in U.S. Pat. No. 5,545,185, discloses a pacemaker for preventing tachyarrhythmia by measuring the cardiac cycle length and detecting the occurrence of a PVC more than a predefined amount between consecutive cycles. When a normal heart beat does not occur within a predefined period of time after such an abrupt change in cycle length, the resulting compensatory pause is eliminated by a cardiac pacer applying an appropriately timed electrical pulse to produce a ventricular depolarization.
Kramer, in U.S. Pat. No. 6,285,907, discloses a cardiac rhythm management system that includes techniques for computing an indicated pacing interval, AV delay, or other timing intervals. The indicated pacing interval is used to time the delivery of biventricular coordination therapy even when ventricular heart rates are irregular, such as in the presence of atrial fibrillation.
Current dual chamber implementations may actually shorten the PAV interval via a rate-adaptive AV response in response to the rate stabilization algorithm, thereby forcing a ventricular pacing output. In addition, a compensatory pause after a PVC temporarily prolongs VA conduction. This pause usually results in a paced atrial and ventricular response. From a ventricular point of view, such an operation is not ideal, because it results in the following sequence: an intrinsic ventricular depolarization using the Purkinje system, a PVC from an ectopic focus in the ventricle, followed by a paced beat usually from a pacing electrode located in the apex of the right ventricle—and, in biventricular pacing, from a fourth focus, an electrode located in the upper posterior portion of the left ventricle. The activation of three or four separate foci within such a short period of time works counter to the maintenance of intrinsic activation. From a clinical point of view, such activation may actually exacerbate the dispersion of refractoriness and/or potential for block/reentry.
In a similar fashion, the sequence described above is not ideal from an atrial point of view. Paced and intrinsic atrial depolarizations produce different hemodynamic responses due to the following activation/contraction coupling sequences. The following sequence may lead to altered hemodynamics such as, for example, atrial sense-ventricular sense (AS-VS), premature AS leading to an intrinsically conducted VS, followed by an atrial pace-ventricular pace (AP-VP). Atrial activation from widely separated foci may detrimentally affect the atrial tissue, making it more prone to atrial fibrillation. Atrial fibrillation, in turn, results in decreased ventricular filling and output. While many paced patients may tolerate such a decrease, the effect on heart failure patients is usually less advantageous.
In view of the above, there is a need for an algorithm that maintains intrinsic, rather than paced, ventricular activity while, at the same time, maintaining various rate stabilization methods, specifically in a dual chamber operation.